Color television system



Patented Dec. '1, 1948 COLOR TELEVISION SYSTEM Constantin S. Szegho, Chicago, Ill., assigner to The Rauland Corporation, Chicago, Ill., a corporation of Illinois Application December 21, 1943, Serial No. 515,081

4 Claims, (Cl. P18-5.4)

This invention relates to cathode ray tubes reproducing a colored image on the screen. More specifically it relates to a television receiving system including a, cathode ray tube and affording color reproduction.

This invention utilizes two phenomena, already knownA in the art, but hitherto not used either separately or in combination for securing color television.

When an electron beam is projected against a mass of solid material such as the fluorescent screen of s, cathode ray tube. it has been found that the extent to which the stream penetrates is a function of the impelling voltages actuating the stream and this function does not vary linearly with the voltage.

The actual distance of penetration likewise varies to a much smaller degree in accordance with the particular material constituting the solid mass, for example the layer of phosphor constituting the fluorescent screen proper.

Certain illustrative examples of the phenomena just described are as follows. In a given cathode ray tube 1-5 kv. were applied to the second anode of the tube and this potential was increased and decreased, respectively, by 33 percent. The corresponding changes occurring in the depth of penetration of the electron beam in this tube were found to be 75 percent and 50 percent repectively. As illustrative of the relatively small effect upon the penetration caused by the use of different phosphors. the following example may be given. Four commonly employed phosphors were compared with zinc sulphide, with respect to penetration, at various potentials of the electron beam, ranging from kv. to 50 kv. The maximum difference in penetration depth for the same potential found between the various phosphors was less than percent.

Another phenomenon already known from the investigations of Lenard is that when an electron stream is projected into a material such as a phosphor, the centers of fluorescence are not distributed along the entire path of the beam through the fphosphor, but are formed mainly` at a certain depth. In other words, an electron beam can penetrate through an outer layer of phosphor without considerable loss of energy and then can give its energy to a layer lying beyond the layer first encountered by the beam. The exhibition of fluorescence will take place substantially entirely at the points where the maximum amount of energy is given up by the beam.

One object of this invention is to provide a cathode ray tube in which the phenomena just described are utilized to cause the production of an image in several discrete colors, the color produced being controlled by varying the potential of the electron beam falling upon a screen coated with several discrete layers of fluorescent materials, each layer exhibiting a different color when excited by the electron beam.

Another object of this invention is to provide a television receiver yielding color images, in which receiver no moving parts are employed.

From researches which have been made, it has ascertained that the depth of penetration of an electron beam increases rapidly when the accelerating potential of the beam is increased. For example when kv. are used, the depth of penetration is about 70 microns. Since the cathode ray tube screens hitherto employed usually have a layer of fluorescent material ranging in thickness from about 5 to 8 microns, it is possible to increase by a. considerable amount the thickness of this uorescent layer, without it being found necessary to employ excessively high accelerating voltages. From these considerations, it can be seen that if several layers of uorescent material be employed, placed successively on top of one another, the total thickness of material so produced will still not reach a value suiiicient to prevent penetration of the electron beam to the undermost layer, even with the employment of moderately high accelerating voltages for the electron beam. If the accelerating voltage of the electron beam be then altered, it will be brought about that the maximum energy loss suffered by the beam can be caused to take place at the particular layer of fluorescent material which will yield a luminous output of the color desired at any particular instant.

From the foregoing considerations it can be seen that color selection according to this invention may be secured without the use of movlng parts, such as rotating color filters and the like, and such selection will be very discriminatory in nature and practically independent of the nature of the various phosphors employed. This invention likewise provides means for varying the velocity of the electron beam without thereby causing any interference with the scanning pattern.

A better understanding of `this invention will be obtained by reference to the appended drawings where:

Fig. 1 shows in section the supporting wall of the fluorescent screen of the cathode ray tube,

having several discrete phosphors arranged thereupon; and

Fig. 2 schematically illustrates a color television receiver embodying this invention.

Referring now to Fig. 1, the outermost layer oi' phosphor is indicated at I0, the electron beam being assumed to be projected first upon this particular layer. Beneath the flrst layer is shown a second layer of phosphor, II. In turn, layer II lies directly over a layer of a third phosphor, I2. Layer I2 is applied directly upon the upper surface of a supporting wall I3, which latter may be made of glass or other suitable material.

Assuming that the relative velocity of the electron beam striking the fluorescent screen may be caused to vary at will, and assuming that the three phosphors shown in Fig. 1 will each yield a different colo'r when excited by the beam, the operation of the fluorescent screen of Fig. 1 will be as follows:

If the velocity of the electron beam be made relatively low, so that the maximum yield of energy therefrom will-take place when it impinges upon outermost layer I0, then this particular layer alone will fluoresce and yield the color characteristic of the chemical com-pound or phosphor forming the layer. If, on the other hand, the velocity of the electron stream be increased so that it penetrates layer I without yielding to the phosphor of this layer any substantial amount of energy. but so that it does yield such energy to the phosphor composing layer II, then layer I I will be excited and will fiuoresce with its characteristic color. Similarly, a still further increase of beam velocity will bring about penetration of both layers l0 and II without substantial energy yield, and allow the excitation of layer I2, the phosphor of which last-mentioned layer will then exhibit its own characteristic color.

Assuming that the luminous output of the fluorescent screen as a Whole is viewed through glass wall I3, there must be taken into consideration the fact that the luminous output of layer II is filtered through layer I2 before becoming visible to an observer. Similarly the luminous output of layer III is subjected to the combined filtering action of layers II and I2. The net resultant color, therefore, when anyl particular layer is excited, may not cover precisely the portions of the spectrum which would represent the luminous output of the phosphor employed in such particular layer, were the phosphor to be employed independently. The possible modifications of color thus brought about can readily be adjusted so as to secure desired results, by applying principles well-known in the optical art, with respect to the order in which the respective layers are arranged, etc.

Furthermore it must be taken into consideration thatthe light emitted by any particular phosphor may excite a secondary fluorescence in the phosphor constituting another layer, and therefore the elemental analytic colors selected in a given case will be influenced by the relative qualities of the light emitted by the particular phosphors directly excited, and those of the light emitted by `any other layer of phosphor thus secondarily excited. There is also the possibility that two or more of the layers of fluorescent material may be simultaneously excited. It is therefore preferred, but not essential, that there be employed phosphors capable of excitation solely by electronic bombardment, and not susceptible to secondary excitation by light incident thereupon.

As one example of a fluorescent screen constructed according to the present invention, let it be assumed that a three-layer screen is used. formed by a zinc silicate fluorescing green, a zinc sulphide fluorescing blue and a zinc cadmium sulphide iluorescing red, and that the zinc cadmium sulphide is activated mainly by electron bombardment. Now assuming that the electron beam velocity is caused to vary, so that the different layers of the screen are selectively excited, and that the colo'r field synchronization signals derived from the receiver will be utilized by means oi' suitable circuits, of which mention will be made later, so as to excite inl turn, for each color-frame, each of the layers or a combination thereof, thereby reproducing the respective red, blue and green light intensities of the transmitted picture, color reproduction will accordingly be secured. That is, the red, blue and green light intensities perceived by the eye, in rapid succession, will reconstruct the original total color effect of the picture as transmitted.

Still further analyzing, by way of example, the functioning of a three-layer screen of the type inst described, it can be further premised that more than one layer is excited by the electron beam and that the succession of layers is the one mentioned before, with the red fluorescing phosphor the last to be reached by the electron beam. Under such assumptions, the mechanism of color` reproduction would be substantially as follows:

At the synchronization signal for red, all three layers will be somewhat excited, but since the electron beam is regulated to excite chiefly the last layer, the excitation centers will be more numerous in the red layer and moreover, as the electrons and photons scattered from the preceding layers will also contribute to an increase of the centers of excitation in the red phosphor, the over-all resultant effect will be a white light with a definite red shade. That is, the effect of exciting all the layers together is substantially the same as if a red shade were mixed with a white light. In order to obtain a blue shade, it will be necessary to have the main loss of energy of the electron beam occur in the layer formed by a blue fluorescing powder, keeping at a minimum the excitation ofthe green phosphor. That can be done by a proper choice of the thickness of the blue phosphor layer. Since the red phosphor is activated only by electrons, the chief possible source of interference is to be found from the green iiuorescing layer, which latter might change the blue to a more greenish shade of blue. The same reasoning applies when a reproduction of a green shade is wanted. The example just given illustrates likewise the fact that a proper choice of the respective phosphors, the proper thickness of the respective layers and the relative position of the various layers with respect to one another, wil1 greatly aid in facilitating the production of a satisfactory reproduced image in color.

Reference is now made to Fig. 2, illustrating a complete television receiver. An antenna 20 picks up the incoming signals and feeds them to the receiver proper 2| by which the various components of the received signals are separated from one another. The components determining the synchronization of the scanning are fed to element 22, where they are transformed into a suitable form so as to control the output of a scanning generator 23. The output of generator 23 is fed to the scanning controls 24 and 25, of cathode ray tube 2l. 'I'hese scanning controls are here illustrated as electro-static deilection plates, respectively controlling the scanning in two dimensions. However, it is to be understood that scanning controls of the electro-magnetic type may be employed if so desired, by making such changes as will be familiar to one skilled in the art and with modiiications hereinafter to be explained.

The components of the received signal representing the luminous intensity are fed from receiver 2| to element 21 where they are transformed ln such fashion as to be susceptible of application to control grid 28 of tube 26. At 29 there is schematically shown the cathode of an electron gun acting as a source for the scanning beam of tube 26 and at l0. 3| is shown an electron lens or the like, for securing proper deilnition of such scanning beam.

Before describing in detail the remaining elements of Fig. 2 and their respective functions, there should` be discussed for purposes of clarity, certain considerations with respect thereto. Assuming that a synchronization signal is derived from the receiver 2|, indicating the particular color to be reproduced at any given instant, the foregoing discussion makes it clear that the velocity of the electron beam must at the same instant be adjusted according to the particular color desired, as for example by altering the ptential of the accelerating electrode 3l in tube 26. Since relatively high potentials ranging from kv. to 50 kv. are used. to accord with the total thickness of the layers of phosphors constituting the screen. it is found convenient to use an aircore high frequency transformer-rectifier circuit.

`The periodical low frequency voltage variations can then beobtained by means of a suitable voltage control circuit, for instance a multi-vibrator controlling the primary current of the transformer by means of a signal of convenient shape and character.

To vary the velocity of the electron beam, there can be applied voltage pulses in synchronism with the color signals to an electrode, such as a ring on the C. R. T. wall between the defiecting devices and the uorescent screen. In this case, howeverl even if the ring electrode be quite near t0 the screen, the potential lines produced by applying a voltage to the ring would spread so far back towards the anode. with a constant potential, that the deilection sensitivity of the deflectors would change, thus causing the pattern on the screen to vary with the applied pulses. Therefore, it can be immediately seen that this particular method of applying the voltage pulses would not be desirable. The dinlculties above described are overcome, in the case of the present invention, by

applying the color voltage pulses directly to the second anode of the electron gun and preventing the change of the scanning pattern by altering the sensitivity of the deilecting system at the same time in any suitable fashion. If the deflection of the beam be obtained by means of electrostatic ilelds, the sensitivity depends upon the deecting voltage in a linear fashion and it will be necessary to apply simultaneously the same relative voltage variations to the second anode and to the deilecting plates, in order that the scanning pattern remain unchanged.

Il, however, it be desired to employ electromagnetic deilection of the cathode ray stream, account must be taken of the fact hat in this case the sensitivity varies inversely with the square root of the applied voltage. Therefore, in order to leave unchanged the scanning pattern, the voltage variations applied to the scanning coils must be proportional to the square root of the voltage variations applied to the accelerating anode. Circuits for securing changes of applying voltage according to this relationship are wellknown in the art and their application to this invention will, accordingly, be 'apparent to one skilled in the art.

Again referring to Fig. 2, the synchronization signals representing color and separated by receiver 2l, are suitably filtered and altered by element I2 which transforms them into a form suitable for voltage-controlling element 33, which latter may be. for example, the multi-vibrator previously described. `The output oi' element Il then functions to control element 34. the local generator of high voltages. 'I'he output of element 34 is applied to accelerating anode 3l. and. at the same time, to the amplitude controls of scan generator 23. so that the scanning may be simultaneously altered, together with the accelerating voltage, in order to avoid changes in the scanning, particularly as previously described. In the foregoing discussion, mention has been made of the fact that the substitution of the electromagnetic scanning controls in lieu of the electrostatic controls, may necessitate the use of additional circuit eiements due to the non-linear relationship of the quantities then involved. Such additional circuit elements are well-known and their connection and operation will be apparent to one skilled in the art.

In the above description and in the claims phosphor denotes a fluorescent material which emits light While excited and for a short period thereafter.

What is claimed is:

1. Color television receiver employing a cathode ray tube and reproducing screen, said screen including a plurality of phosphors arranged in discrete layers thereupon, each phosphor reproducing in the effective luminous output thereof one of the elemental colors used for color analysis, said receiver also including means for scanning- 1y projecting an electron beam upon all the layers of said screen, and means for varying the potential of said beam, thereby selectively to excite a predetermined one of said phosphor layers.

2. Color television system including a transmitter successively scanningly analyzing a color image and transmitting discrete signals corresponding respectively to elemental luminosity, scan synchronization and color synchronization, a receiver including means for receiving and separating said discrete signals, and a cathode ray reproducing tube actuated by said separated signals, said tube including a multi-layer phosphor screen, each layer fluorescing substantially solely in-one of the colors corresponding to said color analysis at said transmitter, an electron gun producing an electron beam cooperating with all the layers of said screen, scanning derlectors actuated by said separated scan synchronization signals, a control grid actuated by said elemental luminosity signals, and an accelerating anode actuated by said color synchronization signals, whereby the velocity and consequent effective screen penetration of said electron beam in said tube is varied in accord with elemental color analysis at said transmitter.

3. System according to claim 2, also including a scan generator controlled by said scan synchronization signals and connected to said deilectors. means for deriving a variable high voltage from said color synchronization signals and 'means for applying said variable high voltage simultaneous- 7 8 ly to said accelerating electrode and to said scan REFERENCES CITED generator' 5 s to compensate for changes m de' The following references are of record in the nection due to changes in beam velocity, whereby me of this patent: color synchronization effects are substantially independent of scanning pattern elects. 5 UNITED STATES PATENTS 4. System according to claim 2, also including Number Name Date voltage control means. actuated by said means 1.934.821 Rudenberg No1/ 14, 1933 for separating said color synchronization signals, 2,096,986 Ardenne Oct. 26, 1937 a generator of high voltages actuated by said volt- 1,177,591 Dawihl et 9,1 oct, 31, 1939 age control means, and means coupling said gen- 10 2,200,285 Lorenzen May 14, 1940 erator to said accelerating anode of said tube. 2,243,828 Leverenz May 27, 1941 2,310,863 Leverenz Feb. 9, 1943 CONSTANTIN B. BZEGHO. 2,330,172 Rosenthal Sept. 21, 1943 

